Conventional vertical wells can create severe coning problems in water drive reservoirs, such as in thin bottom water reservoirs or edgewater reservoirs. Bottom water reservoirs are situated above an aquifer, and there can be a continuous substantially horizontal interface between the reservoir fluid and the aquifer water (water/oil contact). In an edgewater reservoir, only a portion of the reservoir fluid can be substantially in contact with the aquifer water (water/oil contact). Reservoir fluid, comprising hydrocarbons such as but not limited to oil, can be produced from these water drive reservoirs by an expansion of the underlying water and rock, which can force the reservoir fluid into a wellbore. Coning problems can arise because the actual rate of production can exceed the critical rate where the flat surface of water/oil contact begins to deform. Historically, wells producing at critical water-free rates can be less profitable. Horizontal wells have been used to enhance oil production from water drive reservoirs and are typically considered a better alternative than conventional vertical wells as they provide for better economics, improved oil recovery and higher development efficiency. Long horizontal wellbores are able to contact a large reservoir area such that for a given rate, horizontal wells require a lower drawdown, resulting in a less degree of coning/cresting.
Horizontal wells have been employed for enhancing oil recovery from reservoirs having thin oil zones, generally ranging between five and twenty meters, with strong bottom water, such as those found in Bohai Bay of eastern China. To maximize oil production and avoid early water coning or cresting, horizontal wells can be placed near the top of oil sand bodies and wells can be produced with small pressure drawdown before water breakthrough. Nevertheless, the production responses from different horizontal wells can be significantly different from each other even though they are operated under similar conditions. For example, some wells can show premature water coning within a very short time and rapid water cut rising, while others can show later water breakthrough and steady increase of water cut for a longer time.
The existence of thin discontinuous low permeable or impermeable flow barriers with limited horizontal extension or continuity between the wellbore and water/oil contact can impact water coning characteristics. For example, the presence of a flow barrier can be beneficial, as the cumulative water production to produce the same amount of oil can be less and the time required to produce the same amount of oil can be shorter than without the barriers. Additionally, once water reaches the barrier, coning can be limited because the pressure drawdown caused by production can be less at the edge of the barriers than at the well in the absence of the barriers. In some instances, the effects of a completely impermeable barrier on the cone shape can be equivalent to extending the wellbore out to the radius of the barrier.
The productivity of vertical and horizontal wells in formations containing discontinuous shales has been investigated using numerical simulation. For single phase oil flow, the discontinuous shale shows a decrease in the productivity index (or PI) ratio between horizontal and vertical wells. For two-phase oil/water flow in a bottom water reservoir, the randomly distributed discontinuous shales show an increased oil recovery by decreasing water cut in both horizontal and vertical wells (compared with wells without shales). In other words, shales typically shield the horizontal wells from the rising water cone, resulting in lower water cut values. In general, although the total well productivity typically decreases when shales are present, the productivity of oil increases due to the sheltering effect of the shale on water advancement. Accordingly, the long-term effects of discontinuous shales appear to be beneficial with respect to oil production.
The water/oil contact movement in a reservoir containing impermeable layers, where oil can be produced through a horizontal well, has also been investigated using transparent physical 2-D models. Results have shown that increased oil recovery can be obtained when the heel end of a long horizontal well is located above the upper layer of the impermeable streaks. Discontinuous impermeable layers or streaks in a bottom water reservoir act as obstacles to vertical reservoir flow or reduced vertical equivalent permeability. This condition can lead to delayed water breakthrough and significantly improved oil production. Oil production in heterogeneous cases has also shown to be better than in the homogeneous cases, such that they have delayed water breakthrough and slower water cut increases.
Field data has shown that flow barriers benefit horizontal well performance. For example, horizontal wells have been known to produce oil almost one year before the water breakthrough. In light of this, others have suggested to place man-made impermeable barriers around the wellbore to stop the water cone/crest from forming. Others have also suggested using chemicals, such as a polymer, to partially plug bottom water zones in order to improve well production performance in bottom water reservoirs. Others have also recommended drilling long horizontal wells as far from the water/oil contact as possible to improve well performance. However, without the knowledge of physical locations and size of flow barriers, long-term production testing may be needed to obtain reliable pre-development data on the influence of these flow barriers.